I live in an apartment that has plastic bathtubs. My wife & I didn't stay on top of cleaning the tub weekly like we should now we are getting ready to move & want our deposit back. Our bathtubs are very dark & dirty right now. I have tried a lot of different cleaners including kaboom & only seem to get the surface stuff. What can I use to clean a plastic tub? I don't mind elbow grease if it works but as with the way of life now easier is better.